As a means to feed a water slurry comprising a burnable solid, such as pulverized coal or a cellulosic solid waste, to a combustion furnace or gasification reactor, use was made of methods where a slurry is sprayed directly into a combustion furnace or gasification reactor with an aid of a high pressure gas, such as steam or air. The slurry contains water in an amount of from 27 to 80 weight %, relative to the weight of the slurry, and the water vaporizes in a combustion furnace or gasification reactor. In a slurry of pulverized coal and water, the water content is from 27 to 50%, relative to the weight of the slurry. A mixture containing a cellulosic solid waste and water sometimes fails to form a slurry, for instance, when the water content is at most 50% relative to the weight of the mixture. Accordingly, some kinds of slurry require a water content of 50% or more, particularly of from 70 to 80%, relative to the weight of the slurry, depending on the kind of cellulosic solid waste. Therefore, a part of the energy generated in partial combustion of a burnable solid is consumed as latent heat for vaporization of the water, which lowers a temperature in the furnace, resulting in an increase in unburned carbon. In the gasification reactor, fused coal ash deposits because of the lowered temperature in the gasification reactor. This causes troubles such as clog in a withdrawing line for fused ash. In order to prevent the troubles, the temperature in the furnace must be prevented from lowering. Accordingly a larger amount of oxygen than a theoretical amount calculated from an elemental composition of the coal is fed to a gasification reactor in the aforesaid conventional method.
In order to use pulverized coal containing ash of a high fusing temperature, especially in gasification, the inner temperature of a gasification reactor must be maintained at a relatively high temperature. Accordingly, it is difficult in the conventional methods to use coal containing ash of a high fusing temperature. When such a coal containing ash of a high fusing temperature is unavoidably used, an expensive melting point depressant should be used. In addition, a larger amount of oxygen is required to somewhat raise the inner temperature of the gasification reactor whereby melting of the coal ash in the gasification reactor is promoted so as to facilitate removal of the coal ash at a bottom of the gasification reactor and thereby to put the gasification plant in smooth operations. A gasification efficiency of the conventional methods is low due to these factors.
A method of coal gasification by feeding coal and water to a gasification reactor is known, where at least a part of water is fed in a form of steam to the gasification reactor (see Japanese Patent Laid-open No. 2002-155288). According to the method, coal is fed with an aid of steam to a gasification reactor. Therefore, water contained in the mixture of coal and water, preferably the entire amount thereof, is vaporized into steam before being fed to the gasification reactor and, therefore, the above-described drawbacks can be solved.
In the above method, a mixture of solid-liquid system is converted into a mixture of gas-solid or gas-liquid-solid system and fed to a reactor. As equipment by which a slurry of solid-liquid system is fed to a heat exchanger consecutively, heated, converted into a gas-solid system or gas-liquid-solid system, which is then fed to vaporization equipment to recover water, Cracksystem™ is commercially available from Hosokawa Micron Co., Ltd. However, the solvent vaporizes at once in the heat exchanger in this equipment, so that a flow rate of the gas-solid system at an outlet of the heat exchanger exceeds the sonic speed. Accordingly, if the equipment is used for a burnable solid such as coal, heavy abrasion will take place.
In 1979, patent application was filed by the Department of Energy, United States, in which a coal water mixture, CWM, is heated and separated into gas and solid in a flash dryer vessel, and then pulverized coal is fed to a gasification reactor (see U.S. Pat. No. 4,153,427). However, the pulverized coal obtained in the gas-solid separation is not completely dried and, therefore, coagulates, so that a continuous feeding to the gasification reactor is difficult. Accordingly the method has not been put to a practical application.